Hot air from a big fan

David Thomson's paean to Nicole Kidman is a world away from Iain Johnstone's sober biography of Tom Cruise. If only they weren't both so incurious about their subjects' inner lives, says Peter Bradshaw

All movie stars are used to getting letters from obsessive fans: interminable screeds written in capitals in green biro, pressing down very hard on paper torn from ring-bound notebooks - pages and pages of fanatical devotion, unwholesome sexual fantasy and a delusional conviction that there is a genuine relationship between fan and star. Most actors employ staff to screen out these letters, and in some cases enforce court orders to keep their authors at a safe distance.

So how must Hollywood superstar Nicole Kidman feel, now that one of these documents has arrived, not in a crumpled brown envelope - the flap of which has been gummed down God alone knows how - but on three hundred printed pages, between hard covers? Because this is what David Thomson's bizarre and baffling new book is: a loopily swooning tribute to his fave actress - the beautiful, statuesque, Oscar-winning star of The Hours, To Die For and Moulin Rouge

Thomson is too grand to write a muck-raking biography, or even just a fact-raking one, and a standard-issue cuttings job is also infra dig. He has interviewed many people for this book, including the subject herself, whose innocuous remarks are occasionally quoted at length, but there is nothing of what you might call original research here; there are no juicy stories or anecdotes. No, this book is simply a rhapsody on the theme of how David Thomson feels about Nicole Kidman, the Nicole Kidman he worships, and I do mean worships, in the dark of the cinema auditorium.

It is a cadenza of eccentric connoisseurship, subtly but pointedly refusing the subordinate status of journalist or biographer; the author appears to be wistfully auditioning to be Nicole's male co-star, or at the very least the male version of her Ugly Best Friend. Thomson writes things such as: "I suspect she is as fragrant as spring, as ripe as summer, as sad as autumn, and as coldly possessed as winter." (If only Kenneth Williams were alive to do the audio book.) He unblushingly intuits how Kidman feels at various triumphant moments of her career: "She is exuberant; she feels her power ..."

On the same intuitive basis, Thomson does not hesitate to denigrate actresses who are not Kidman. He will not be winning any gallantry medals for this observation about Elisabeth Shue, whose career did not flourish after Leaving Las Vegas: "Why? Well, she is 40 this year, and I suspect she had a weight problem that was difficult to control." Really? In the absence of hard facts, or a compelling reason for raising the subject in the first place, it might be better for him to keep his suspicions to himself. Certainly if he and Shue find themselves at the same Hollywood party, and the author is poolside, with his back to her - well, she might not be able to resist the temptation.

And yet, and yet. Amid all the gibbering, there are nuggets of insight. Thomson is like an experimental jazz sax player, who occasionally finds a beautiful phrase - then it's back to a couple more hours of squeaking and honking. Thomson has funny passages in which he fantasises about Kidman in a remake of Rebecca, and actually regales us with a dream he once had about being in the movie Belle de Jour, where Kidman is playing Catherine Deneuve's maid. Thomson has the grace to be reasonably embarrassed about Kidman's silly false nose playing Virginia Woolf in The Hours, and he makes a bold and generous case for her performance in Jez Butterworth's comedy Birthday Girl.

Most importantly, he discusses her appearance in Jonathan Glazer's thriller Birth, in which Kidman plays a widow confronted by a small boy claiming to be the reincarnation of her late husband. Famously, that movie had an extended single shot of her beautiful, stricken face, listening to Wagner at a concert hall, as she absorbs the possibility that the boy is telling the truth. It is a brilliant moment, showing Kidman at her elegant best, and just right for Thomson's mode of visionary adoration. It would have been audacious and exciting for him to have written a whole chapter, or maybe a whole book, about Birth, or about just that legendary shot. Instead, exasperatingly, he gives it hardly more weight than all the boring details about Kidman's various cover shoots for glossy magazines.

No book about Kidman can avoid the enigma of her marriage to Tom Cruise - in which the couple adopted children when she was only in her mid-20s - and the mystery of their painful divorce, when she was three months pregnant, followed by a miscarriage. Infidelity might have been involved. Or it could have been that Stanley Kubrick himself, by putting the couple through a long and difficult movie about suspected infidelity in Eyes Wide Shut, opened fatal psychological wounds. Who knows? Certainly not David Thomson, but it doesn't stop him from doing a little indelicate noodling on the subject, and even recycling what appears to be scurrilous supermarket-tabloid gossip to the effect that Kidman offered Cruise a DNA paternity test on her miscarried foetus.

Iain Johnstone's biography of Cruise, by contrast, is an unpretentious model of clarity and humility, simply rounding up the known facts - although, unfortunately, his book was sent to the printers just before Paramount Pictures sensationally terminated their relationship with Cruise, citing his controversial behaviour and opinions. Johnstone duly discusses the media rows, the Scientology, the TV sofa-bouncing - though he steers clear of the cheeky South Park TV skit "Trapped in the Closet". He pays due tribute to Cruise's remarkable career: the 1980s pretty-boy who became a global megastar, heavy-hitting producer and outstanding character actor. All of this is presented neutrally, and Johnstone doesn't see it as his job to offer strong opinions, or really any opinions. It is Thomson who, for all his waywardness, correctly identifies Cruise's finest hour as his performance as Frank Mackey, the sinister male-seduction coach in Paul Thomas Anderson's ensemble movie Magnolia.

Yet both Thomson and Johnstone are just so weirdly incurious about their subjects' inner lives, incurious about what it is like to be them, profoundly reluctant to believe in them as flesh-and-blood people, the narrative arc of whose actual lives might be quite different from that suggested by the roles they have played, the interviews they have given, the magazine covers they have done. In their different ways, these courtiers protect their sovereigns' magic against the letting in of daylight.

· Peter Bradshaw's novel Dr Sweet and His Daughter is published by Picador